1. Field of the Invention
One embodiment of the present invention relates to a dibenzocarbazole compound. One embodiment of the present invention also relates to a light-emitting element, and a light-emitting device, a display device, an electronic device, and a lighting device each including the light-emitting element.
Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a lighting device, a power storage device, a memory device, a method for driving any of them, and a method for manufacturing any of them.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting material (an EL layer) is interposed between a pair of electrodes. By applying a voltage between the pair of electrodes of this element, light emission from the light-emitting material can be obtained.
Since the above light-emitting element is of a self-luminous type, a display device using this light-emitting element has advantages such as high visibility, no necessity of a backlight, low power consumption, and the like. Furthermore, the display device also has advantages in that it can be formed to be thin and lightweight, and has high response speed.
In a light-emitting element (e.g., an organic EL element) whose EL layer contains an organic material as a light-emitting material and is provided between a pair of electrodes, application of a voltage between the pair of electrodes causes injection of electrons from a cathode and holes from an anode into the EL layer having a light-emitting property and thus a current flows. By recombination of the injected electrons and holes, the organic material having a light-emitting property is brought into an excited state to provide light emission.
Note that an excited state formed by an organic material can be a singlet excited state (S*) or a triplet excited state (T*). Light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence. The statistical generation ratio of the excited states in the light-emitting element is considered to be S*:T*=1:3. In other words, a light-emitting element including a material that emits phosphorescence (a phosphorescent material) has higher emission efficiency than a light-emitting element including a material that emits fluorescence (a fluorescent material). Therefore, light-emitting elements containing phosphorescent materials capable of converting energy of the triplet excited state into light emission have been actively developed in recent years.
Among light-emitting elements including phosphorescent materials, a light-emitting element that emits blue light has not been put into practical use yet because it is difficult to develop a stable material having a high triplet excitation energy level. For this reason, a light-emitting element including a more stable fluorescent material has been developed for a light-emitting element that emits blue light and a material for increasing the emission efficiency and lifetime of a light-emitting element including a fluorescent material has been searched.
Note that the performance of a light-emitting element, such as emission efficiency or lifetime, is significantly affected by not only the performance of a light-emitting material but also the performance of a host material for exciting the light-emitting material or a carrier-transport material for transporting a carrier. Therefore, materials having a variety of molecular structures have been proposed in order to increase the emission efficiency and the lifetime of a light-emitting element (for example, Patent Documents 1 and 2).
In particular, as for a light-emitting element that emits deep blue light, not only a light-emitting material but also a host material for exciting the light-emitting material needs to have high excitation energy. Accordingly, development of a highly stable host material which can excite a light-emitting material efficiently has been required.